The results clearly demonstrated that the marketable yield responses of broccoli closely matched the responses in soil and crop nutrient, crop growth and biomass accumulations. Considering the marketable yields, there was no significant yield difference between cover cropping and fallow treatments for the first year cropping . During this year, the number of marketable heads and fresh weights of the marketable heads from the first, second and total crop harvest were not significantly different from each other for all cropping treatments .Differences between cropping treatments in vegetable marketable yield commenced in the second year cropping year. Interestingly, broccoli gain from cover crops even for the second year study was only with fresh weights of the marketable heads, but not the number of heads . Higher fresh broccoli marketable heads were observed from the first and second harvest from the 2008 crops. Broccoli crops produced higher number of marketable heads and fresh weights of the marketable heads during the third year . The total number and fresh weights of marketable heads from the two harvest periods of crops from a summer cowpea plots for 2008 and 2009 were about 36% and 48% higher, respectively compared to these grown on the summer fallow . The findings in general, suggest the long-term buildup and additive effects of cover cropping rotations on the subsequent vegetable crop.The cover cropping treatments increased soil organic matter contents within the subsequent vegetable crop. However, statistically significant differences in soil organic matter component of the soil was not detected until at the broccoli harvest time of 2008 and following cover crop incorporation in 2009. Since these samplings were both after cover crop or broccoli incorporation, grow rack the higher soil organic matter contents must have been from the decomposition of the cover crop residues as well as broccoli.

A continued practice of cover cropping becomes an investment in building healthy soil over the long term, builds organic matter and by serving as food source to soil organisms , and increasing soil productivity . The initial year similarity in organic matter content levels of cover cropped and fallow plots is probably due to the fact that soil organic matter buildup takes place very slowly.Organic matter of a soil is important in improving soil structure, increase infiltration and cation exchange capacity and serves as efficient storage of nutrients . Upon its breakdown soil organic matter releases available nutrients to plants . However, soil contents of organic matter frequency and type of cultivation , cropping and residue management , or fertilizer N input may also affect soil nutrient status. The soil organic matter contents from cropping treatments were reflected in variation of some soil nutrient contents. As has been shown from soil nutrient analysis, nutrient enhancement from cover cropping was more visible following cover crop residue incorporation. Wagger and Creamer and Baldwin suggest that higher contents of soil nutrients were associated either manure applications or cover crop incorporations. While soil nutrient concentrations oscillated between sampling periods and years, Ca and Na concentrations and soil cation exchange capacities were higher for the cover crop treatments of the second year ABH sampling and at ACCP sampling in the third year. The higher soil nutrient concentration and CEC from the cover crop plots of the 2008 must have been from the accumulation from the previous year crop residue decomposition . However, most of the soil nutrient concentration right after cover crop incorporation of 2008 was not different among the cropping treatments, indicating the probability of nutrient immobilization following residue incorporations.

The latter increase in soil nutrition must have been from the mineralization process following cover crop residue decomposition. The trend suggests that it is possible to buildup up soil nutrient contents with the use of summer cover cropping and allow the subsequent vegetable crop to make use of accumulated soil nutrients. It also suggests that the process of cover cropping rotations must be continuous in order to achieve a continuous improvement in nutrient availability for the subsequent vegetable crop. Similarly, soil NO3 was consistently higher for the cover crop treatments relative to the summer fallow, but not until after cover crop incorporation of 2008. Soil NO3 level declined and was not different among the cropping treatments at ABH sampling of 2009. The decline in NO3 at broccoli harvest was probably depletion due to nitrogen uptake by broccoli. In relatively higher soil NO3 levels in 2009 than in 2008, suggests a nutrient build-up effect from repeated cover cropping and a higher N mineralization with increased years of residue accumulation. Soil SO4, and percent cation saturations were higher for the cover crop treatments, compared to the fallow, but not until 2009. Mn and B were higher in the fallow than in the cover cropped plots at harvest. My results demonstrated the importance of preceding cultivation of vegetable crops with summer cover cropping instead of leaving the land fallow. Following broccoli production after summer cover cropping benefitted the crop in enhancing and increasing soil nutrient availability, enhancing crop growth and marketable yield. The ultimate benefit of cover cropping may also come from pest suppression, enhancing beneficial organisms, increased biodiversity and other indirect benefits of cover cropping. Since soil nutrition is particularly critical for organic food production practices, the use of cover crops could help fulfill this need. I observed that not all soil nutrients are equally enhanced with the use of cover cropping. Besides, not all cover crops are equal contributors to added soil nutrition.

Increases in soil nutrient content, particularly soil NO3 was greater when the cover crop was cowpea than when it was a marigold, probably relating to the nitrogen fixing capability of cowpea. Leguminous cover crops with a biological nitrogen-fixing capability play a much more important role and may reduce dependence of the subsequent crop on synthetic nitrogen fertilizers . However, Franzluebbers et al. 1994; Fageria et al. 2005 all suggest that N supply from the decomposing residues must coincide with the subsequent crop N demand and proper management of residue in order to provide increased efficiency of cover crop use. The N supply from legumes could reduce N application rates below the recommended rate for subsequent vegetable crops . The contribution of N is the primary benefit of leguminous crops resulting in increased crop yields . Therefore, my findings of variable nutrient contribution from different cover crops suggest that the extent of soil nutrient build up is dependent on the type of the cover crops and that proper cover crop compatibility and selection be made based on the requirement of a farm and residue management practices. Although legumes could release fixed N to the soil, leguminous cover crop residues may also transport a large portion of their biomass nitrogen into the seeds if allowed to flower and mature, because the N-fixing symbiosis of the legume shuts down when the crop stops active growth. Therefore, a good management that benefits the subsequent vegetable crop is to kill the legume cover crops in the early- to mid-blossom stage and plant the following cash crop without delay, aside from any period for residue decomposition . Since soil nutrition is somewhat related to soil organic matter accumulations, such benefits must depend on a balanced interaction of organic matter, soil organisms that break down crop residues and nutrient cycling and selection of the cover crop and residue management practices . The increased microbial immobilization of soluble N may require modified fertility management practices that increases nutrient availability to coincide with plant demand . Immobilized nutrients may be subsequently available through mineralization after incorporation . On the other hand, the pattern and timing of mineralization of nutrients depends on the residue quality, soil type, temperature, soil moisture content and timing and method of incorporation . The higher soil Ca and Na under cover crop treatments may also be due to the fact that cover crops may help bring nutrients such as calcium and potassium back into the upper soil profile from deep soil layers and then release them back into the active organic matter when they die and decompose . As for the soil contents, higher N, Mg and Na were detected in the shoots of broccoli grown on the summer cowpea plots compared to the fallow treatments. However, vertical racks these nutrient increases were only in the 2009 crops, but not the 2008, indicating a need for repetitive and multiple-year cover cropping rotations to provide increased nutrient supply to the subsequent vegetable crop. In some cases, while some soil nutrients were higher for the cover crop treatments than in the bare soil, the subsequent crop does not seem to have made full benefit of the improved soil nutrition. My findings were consistent with Baggs et al. where no significant effect of cover cropping was observed on the N content or yield of the subsequent oats crop, regardless of the release of N from decomposing cover crop tissues.

These observations were attributed to non-limiting N in this soil for any benefits to become apparent immediately. It is also possible that mineralization of some nutrients from incorporated residues may be delayed , resulting in conflicting evidence over the ‗fertilizer value‘ of cover crops showed that recovery of N by the subsequent crop is typically less than 30–40%. Cover crops may also reduce available soil NO3 compared with the fallow treatment by 18–44% as a result of low mineralization rates.My observation of low nutrient content in shoots of crops and the many other previous findings suggest that crop nutrient contents do not necessarily match soil N contents. Baggs et al. showed that crop N alone is an adequate indicator of the quality of a cover crop. In some cases a higher N content in crops was observed following a bare ground treatment than the cover crops, suggesting that N was not available for crop uptake following cover crop incorporation and may be delayed until after complete mineralization . Nutrient immobilization from incorporation of residues is short-lived immobilization for soils with comparatively high C:N ratio . Cover crops can provide N to subsequent crops in two ways 1) non-legume cover crops recover and recycle residual fertilizer N, and 2) legume cover crops fix atmospheric N for the later crops . In general while cover crops have the potential to supply nutrients to the subsequent crops, synchronization of N supply from decomposing residues and crop nutrient demands must govern the timing of cover crop kill Creamer and Baldwin, 2000. If not properly managed cover crops create nutrient deficiency as a result of immobilization . This is probably the reason why Schroeder et al. rejected the use of cowpea crop residues as fertilizer N inputs for broccoli. Consistent with nutrient status, crop height growth was highest for those from cowpea, followed by marigold and least for crops grown on the summer fallow. The increase in height of broccoli grown on the cover crops is more prominent after the third week of sampling for all study years, but no height differences were observed between cropping treatments for the initial growth stages . This initial stage indifference in crop height could be due to a growth lag phase and that crops are not able to make immediate use of the added resources. Broccoli canopy spread was similar to the crop‘s height responses in that broccoli on the summer cover crop treatments for all years were relatively of broader canopy, but were most significant for the 2008 cropping year. Canopy growth differences between the study years may have been due to the variation in weather conditions of the different experimentation years. Mean leaf number production and variation between cropping treatments were clearly visible for the 2008 and 2009 cropping seasons than for the 2007 crops. These visible increases in number of broccoli leaves with increasing cover cropping rotations indicate the benefits of multiple cover cropping rations and their buildup effects with increasing use of the system. Regardless of some differences in various growth progressions of the vegetable crop, there were some similarities in their responses to the cropping system treatments. First, crop growth is most enhanced by preceding it with summer cowpea than marigold. Secondly, the taller and the greater the canopy spread of the crops are, the higher are the number of leaves per plant.